This invention relates to a video special effect producing system used in television studios, and more particularly to a system for controlling the phase or the peak position of a parabolic wave used to produce a circular or elliptical montage pattern on the screen of a television cathode ray tube.
As one example of video special effects a so-called wipe or montage effect is known in which is inserted in part of the television screen a picture which is different from a picture on the remaining part of the screen. Generation of a switching signal or keying signal for combining two kinds of video signals into one composite video signal is required to provide such video special effect. In a conventional special effect producing system for providing a typical circular montage pattern horizontal and vertical parabolic waves are generated at the horizontal line-scanning frequency and the vertical field-scanning frequency by horizontal and vertical drive pulses, and then mixed together. The mixed output of the horizontal and vertical parabolic waves is sliced by a slicer to produce a keying signal. When the horizontal and vertical parabolic waves each have a symmetric waveform a keying signal is generated which provides a circular wipe pattern centered at the center of screen. The size of the circular wipe pattern to at the center of screen. The size of the circular wipe is controlled by changing the slice level of the slicer by means of a fader operating device. The wipe pattern is moved up and down, and right and left, by adjusting a relative phase or a time relation between the horizontal and vertical drive pulses by means of position controllers connected to phase adjusting circuits for the horizontal and vertical drive pulses. The phase or the peak position of the parabolic wave so obtained is displaced with respect to the phase or the peak position of the parabolic wave for providing a circular wipe pattern centered at the center of screen.
In the above-mentioned special effect producing system in which the wipe pattern is moved by the phase adjustment of the horizontal and vertical drive pulses, when the center position of wipe pattern is shifted from the center of the screen, if the pattern is enlarged by the fader operating device, an unwanted twin pattern appears at the side opposite to that on which a desired pattern appears. In order to prevent such an unwanted twin pattern a waveform process or a twin pattern inhibiting process is usually effected which removes part of parabolic waveform of the horizontal parabolic wave and/or the vertical parabolic wave within each cycle period thereof. If the discontinuous parabolic wave obtained by such a waveform process is used, a distortion such as to make part of a circular pattern a straight line will occur dependent upon the position and size of the circular pattern. And further, when the pattern is positioned far away from the center of the screen, a complete change-over of screen from one picture image to another by means of fader control can not be achieved.
An elliptical pattern is obtained by changing the relative amplitude of the horizontal and vertical parabolic waves. The above-mentioned problems on the circular pattern also occur for the elliptical pattern.
In order to settle the above-mentioned problems it is necessary to provide horizontal and vertical parabolic waves whose instantaneous amplitude continuously varies during a substantially one cycle period even if the peak of the parabolic wave is at any point of time within one cycle period.